1. Field of the Invention
The present invention relates to a lithographic mask capable of providing regions of graded exposure, such that when combined with anisotropic etching provides three dimensional microstructures of arbitrary size and shape (generally non-prismatic). The present invention also relates to a method for producing a gray-scale x-ray mask and fabricating a tool useful in the preparation of plated molds, or embossing stamps, for printing fine structures into plastic substrates.
2. Description of Related Art
A variety of methods are presently known for making microparts. In particular, U.S. Pat. Ser. No. 5,501,893 to Laermer, et al., describe a lithographic technique for etching silicon, generally referred to as xe2x80x9canisotropic etching,xe2x80x9d where it is possible to achieve deeply-extending trenches while simultaneously providing side walls which are as nearly parallel and vertical as desired. In particular, Laermer, et al., (""893) teach a multi-step, two stage process for alternately etching an exposed silicon surface in a reactive ion plasma followed by coating the etched surfaces with a thin polymerized layer, wherein the polymer coating serves to protect the wall surfaces of the trench from action of the plasma.
Commonly owned U.S. patent application Ser. No. 09/636,002, filed Aug. 9, 2000, now U.S. Pat. No. 6,477,225, and herein incorporated by reference, discloses a method for preparing an x-ray mask prepared by utilizing an anisotropic deep etching technique. The x-ray mask of the ""002 application discloses an x-ray absorbing structure incorporated into the bulk thickness of a standard silicon wafer to provide a robust processing mask capable of withstanding repeated handling.
Finally, U.S. Pat. No. 6,075,650 discloses to prepare a photoresist with a spatially variable dose of electromagnetic radiation to create a surface-relief structures upon developing the exposed resist. The ""650 patent also discloses a gray scale mask exposure method, and methods incorporating moving apertures. Not disclosed, however, is a description or suggestion for creating a metal stamping tool by utilizing an etched substrate as a plating base.
It is known that a standard lithographic mask is an essentially binary tool for imprinting an image into the cross section of a thin uniform photosensitive layer covering a substrate. (The mask is described as xe2x80x9cbinaryxe2x80x9d in that radiation from a light source either reaches the surface of the photosensitive layer or is xe2x80x9cmaskedxe2x80x9d or blocked and prevented from reaching that surface.) When such an image is developed, the resultant pattern comprises a two dimensional replication of the mask image and if the pattern is transferred into the substrate by etching the result yields features having the same depth/height.
More recently, commercial xe2x80x9cgray-scalexe2x80x9d masks have become available that provide graded exposure in individual feature within patterns of features. Such masks are, however, quite fragile and difficult to handle.
What is needed is a simple method for providing microstructures that smoothly vary in height from point-to-point in etched substrates, i.e., structure which can vary in all three dimensions. Also needed is a simple, so-called xe2x80x9cgray-scalexe2x80x9d mask wherein the mask structures have variable attenuating properties from point-to-point across the mask surface. Such a mask could be used to provide etched features in silicon having arbitrary three dimensional shapes which could themselves be used as a final device or as a plating base for providing a metal embossing stamp.
It is, therefore, an object of this invention is to provide a silicon wafer having arbitrary three dimensional microstructures etched into a surface of the wafer.
Another object of this invention is to provide a process for fabricating highly accurate, three dimensional metal embossing tools.
Yet another object of the invention is to provide an x-ray mask having an embedded metal pattern whose thickness varies from point-to-point to provide varying amounts of attenuation of x-ray radiation striking the pattern in a direction parallel to the metal thickness.
Yet another object of this invention is to provide a simple method for fabricating a gray scale mask useful for ultraviolet and for x-ray radiation.